Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. High voltage discharges between two electrodes excite a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light, which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.
Excimer lasers are a particular type of electric gas discharge laser and have been known as such since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled xe2x80x9cCompact Excimer Laser.xe2x80x9d This patent has been assigned to Applicants"" employer, and the patent is hereby incorporated herein by reference. The excimer laser described in patent ""884 is a high repetition rate pulse laser. In FIG. 1 and FIG. 2, the principal elements of the laser 10 are shown. (FIG. 1 corresponds to FIG. 1 and FIG. 2 corresponds to FIG. 7 in patent ""884.) The discharges are between two long (about 23 inches) electrodes 18 and 20 spaced apart by about ⅝ inch. Repetition rates of prior art lasers, like the one described are typically within the range of about 100 to 2000 pulses per second. These high repetition rate lasers are usually provided with a gas circulation system. In the above referred to laser, this is done with a long squirrel-cage type fan 46, having about 23 blades 48. The fan blade structure is slightly longer than the electrodes 18 and 20 and provides sufficient circulation so that at operating pulse rates, the discharge disturbed gas between the electrodes is cleared between pulses. A finned water-cooled heat exchanger 58 in FIG. 1 is used to remove heat from the laser gas which is added by the discharges and the fan.
These excimer lasers, when used for integrated circuit lithography, are typically operated on a fabrication line xe2x80x9caround-the-clockxe2x80x9d; therefore, down time can be expensive. For this reason most of the components are organized into modules which can be replaced normally within a few minutes.
Excimer lasers used for lithography must have its output beam reduced in bandwidth to a fraction of a picometer. This xe2x80x9cline-narrowingxe2x80x9d is typically accomplished in a line narrowing module (called a xe2x80x9cline narrowing packagexe2x80x9d or xe2x80x9cLNPxe2x80x9d) which forms the back of the laser""s resonant cavity. This LNP typically is comprised of delicate optical elements including prisms, a mirror and a grating.
When used as a light source for integrated circuit lithography, the laser beam parameters (i.e., pulse energy, wavelength and bandwidth) typically are controlled to within very tight specifications. This requires pulse-to-pulse feedback control of pulse energy and somewhat slower feedback control of the wavelength of the line narrowed output beam. Wavelength and bandwidth measurements are made using gratings and etalons to produce spectral patterns on photodiode arrays. A doubling or more of the pulse rate requires these feedback control systems to perform much faster.
A need exists for gas discharge laser light sources operating at higher average power than prior art devices in order to facilitate increases in production of integrated circuits. One method of increasing average power is to increase the pulse repetition rate to 4000 Hz and greater while maintaining pulse energies in the range of 5-10 mJ. Another method is to increase the pulse energy. Higher repetition rates and/or increased pulse energies create both thermal and radiation challenges inside and downstream of the resonant cavity of these gas discharge lasers especially with respect to delicate optical instruments such as gratings and etalons.
When high-energy ultraviolet beams, such as 248 nm, 193 nm and 157 nm laser beam pass through air, the photons excite atoms and molecules in the air. These excited molecules and atoms can plate out on sensitive optical components or degrade optical coatings. A known technique to minimize this problem is to enclose and purge the beam path with a purge gas such as nitrogen.
Another prior art reason for purging the beam path especially for the 193 nm and 157 nm lasers, is to eliminate oxygen and other absorbers from the air. Oxygen is a very strong absorber of 157 nm light and a strong absorber of 193 nm light.
What is needed are improvements in the components of these gas discharge lasers to permit high quality performance at these substantially increased average power levels.
The present invention provides a wavemeter for an ultraviolet laser capable of long life beam quality monitoring in a pulsed ultraviolet laser system at pulse rates greater than 2000 Hz at pulse energies at 5 mJ or greater. In a preferred embodiment an enhanced illumination configuration reduces per pulse illumination of an etalon by a factor of 28 compared to a popular prior art configuration. Optics are provided in this embodiment which reduce light entering the etalon to only that amount needed to illuminate a linear photo diode array positioned to measure interference patterns produced by the etalon. In this preferred embodiment two sample beams produced by reflections from two surfaces of a beam splitter are diffused by a defractive diffuser and the output of the defractive diffuser is focused on two separate secondary diffusers effectively combining both beams in two separate spectrally equivalent diffuse beams. One beam is used for wavelength and bandwidth measurement and the other beam is used for calibration. In preferred embodiments an etalon chamber contains nitrogen with an oxygen concentration of between 1.6 and 2.4 percent.